Field of the Invention
The present invention relates in general to a motor vehicle control system including a differential control device for controlling the operating state of a differential gear associated with a plurality of wheels of the vehicle, and a wheel rotation control device for controlling the rotating state of the wheels. More particularly, the invention is concerned with techniques for correct determination as to whether an operation of the wheel rotation control device should be permitted or inhibited, depending upon the operating state of the differential gear or differential control device.
Discussion of the Related Art
JP-A-6-297975 disclosed an example of such motor vehicle control system including both a differential control device and a wheel rotation control device.
Generally, the differential control device includes (a) a differential actuator for operating a differential connected to a plurality of wheels of the vehicle, and (b) a differential controller for controlling the differential actuator to operate the differential for thereby controlling the differential actions of the wheels.
The differential control device may be adapted to selectively place the differential in one of a LOCK state in which the differential actions of the wheels are substantially inhibited, and a FREE state in which the differential actions are substantially permitted. The differential control device may include operator-controlled input means which is operated to select the FREE or LOCK state of the differential, so that the differential control device operates to selectively place the differential in the FREE or LOCK state.
On the other hand, the wheel rotation control device generally includes (a) slip detecting means for detecting an amount relating to slipping of the wheels, (b) an actuator (e.g., hydraulic pressure control device electrically operated to regulate braking pressure applied to each wheel) for controlling the rotating state of the wheel, and (c) a controller for controlling the actuator to control the rotating state of the wheel, according to an output of the slip detecting means.
The above-identified publication JP-A-6-297975 shows an example of the wheel rotation control device in the form of an anti-lock brake pressure control device, which is adapted to control a braking torque of the wheel so as to prevent locking of the wheel during brake application to the vehicle. For example, the braking torque may be controlled by regulating the hydraulic braking pressure to be applied to the wheel. Other examples of the wheel rotation control device include a traction control device and a yaw moment control device. The traction control device is arranged to control a driving torque of each drive wheel of the vehicle so as to prevent spinning or slipping of the drive wheels. For instance, the driving torque of the drive wheel may be controlled by controlling the operating speed of a vehicle engine or by regulating the hydraulic braking pressure to be applied to the drive wheel. The yaw moment control device is adapted to brake right and left wheels of the vehicle such that the braking forces acting on the right and left wheels are made different from each other to thereby control a yaw moment of the vehicle for stabilizing the attitude of the running vehicle.
On some motor vehicles, brake application is effected while the differential actions between front and rear wheels are substantially restricted or inhibited. In this case, the rotating states of the front wheels and those of the rear wheels may not be controlled independently of each other, whereby the wheel rotation control device may fail to correctly control the rotating states of both of the front and rear wheels. Where brake application is effected while the differential actions between right and left wheels are substantially restricted or inhibited, the rotating states of the right and left wheels may not be controlled independently of each other, whereby the wheel rotation control device may fail to correctly control the rotating states of both of the right and left wheels. However, there is a strong need for correct control of the rotating states of the right and left rear wheels by the wheel rotation control device, during brake application on an uneven friction-coefficient road surface whose right and left areas have different friction coefficient values, particularly where the brake application is effected while not only the differential actions between the front and rear wheels but also the differential actions between the right and left wheels are substantially inhibited. In this respect, it is noted that the right and left rear wheels generally contribute to improved running stability of the vehicle during brake application to a greater extent than the front wheels.
In the light of the above drawback, the above-identified publication proposes techniques for preventing abnormal operation of the anti-lock brake pressure control device in the case where the differential control device and the anti-lock brake pressure control device are concurrently operated for the front and rear wheels, more particularly, techniques for reducing a target value of the amount of restriction of the differential actions of the front and rear wheels by the differential control device in the above case, so that the anti-lock brake pressure control device is operated in the normal manner while the differential is placed in a state in which the differential actions of the front and rear wheels are permitted to a greater extent in the above case. The same publication also discloses techniques for totally or partially disabling the anti-lock brake pressure control device in the case where the amount of restriction of the differential actions cannot be reduced due to some defect in the differential control device.
The present inventor developed a differential control device of the type described below. This differential control device includes the differential actuator and controller discussed above, and further includes operator-controlled input means and vehicle condition detecting means, so that the controller receives an output signal of the input means indicating the vehicle operator's desire to place the differential in the LOCK state, and an output signal of the vehicle condition detecting means indicative of the running condition of the vehicle. The controller is adapted to automatically inhibit the differential from being placed in the LOCK state even when the operator-controlled input means has been operated to place the differential in the LOCK state, if the vehicle is in a predetermined running condition. In this respect, it is noted that the stability of turning of the vehicle at a relatively high speed is lowered when the differential is placed in the LOCK state. In this sense, it is desirable that the operation of the differential to establish the LOCK state be effected only when the vehicle running speed is relatively low. The controller may be arranged to automatically inhibit the differential from being placed in the LOCK state if the vehicle operator has operated the input means to establish the LOCK state during running of the vehicle at a relatively high speed. This arrangement assures improved turning stability of the vehicle at a high speed without the differential being placed in the LOCK state.
Thus, the differential control device developed by the present inventor does not always assure coincidence between the operating state of the differential desired by the operator and the actually established operating state of the differential. Namely, under the predetermined vehicle running condition, the differential control device ignores the operator's intention (i.e., intention to establish the LOCK state) as represented by the output signal of the operator-controlled input means. In the vehicle control system including the wheel rotation control device as well as the differential control device developed by the present inventor, an operation of the wheel rotation control device may be undesirably inhibited even with the differential actually kept in the FREE state, if the operating state of the differential is determined on the basis of the output signal of the operator-controlled input means. However, the wheel rotation control device should be operated if the differential remains in the FREE state because the operator-controlled input means is operated under the predetermined running condition.
In view of the above fact in the vehicle control system including the differential control device and the wheel rotation control device, it is desirable to actually detect the currently established operating state (LOCK or FREE state) of the differential, and effect the determination as to whether the operation of the wheel rotation control device should be permitted or inhibited, depending upon the detected operating state of the differential, so that the wheel rotation control device is operated as long as the differential is placed in the FREE state, irrespective of the operator's intention as represented by the output signal of the input means.
The operating state of the differential as represented by the operator-controlled input means and the actually established operating state do not coincide with each other, in some other instances, for example, in a transient state of the differential, more specifically, during a time period between the moment at which the differential control device receives the output signal of the input means indicative of the operator's intention to establish the LOCK state of the differential, and the moment at which the differential has been actually brought to the LOCK state. The differential cannot be brought to the LOCK state immediately after the generation of the output signal of the input means, and remains in the FREE state for some time even in the presence of the output signal. The detection of the FREE state in this transient state indicates that the differential will be brought to the LOCK state in some time, and therefore the operation of the wheel rotation control device is desirably permitted since the operation of this device is not desirable when the differential is subsequently placed in the LOCK state.
There will be explained in detail one of the reasons why it is desirable to determine that the operation of the wheel rotation control device should be inhibited, when the transient state of the differential is detected.
The vehicle control system may be arranged as described below, for example. That is, the differential control device is designed to place the differential in the LOCK state in response to the output signal of the operator-controlled input means indicative of the operator's intention to establish the LOCK state. On the other hand, the wheel rotation control device is designed to be inhibited from operating when the differential is placed in the LOCK state. Further, the differential control device includes (a) differential detecting means for determining whether the differential is placed in the LOCK state or not, (b) determining means for determining that the operation of the wheel rotation control device should be inhibited, if the differential is placed in the LOCK state, and (c) indicator means for providing an indication that the operation of the wheel rotation control device is inhibited, if the determining means determines that the operation of the wheel rotation control device should be inhibited.
In the vehicle control system designed as described above, the indicator means informs the vehicle operator that the operation of the wheel rotation control device is inhibited, in quick response to the output signal of the operator-controlled input means, if the differential has been brought to the LOCK state in quick response to the output signal. However, the differential may be held in the transient state some time after the generation of the output signal of the input means. In this case, the indicator means does not provide an indication that the operation of the wheel rotation control device is inhibited, in quick response to the operator's operation of the input means, whereby the operator waits for the completion of an operation of the differential control device to place the differential in the LOCK state. That is, the indication that the operation of the wheel rotation control device is inhibited is delayed with respect to the moment at which the input means is operated. However, it is desirable to design the vehicle control device such that the indicator means informs the operator of the disabling of the wheel rotation control device, in quick response to the output signal of the input means, irrespective of whether the differential is actually in the transient state or not.
For the above reason, it is desirable to determine that the operation of the wheel rotation control device should be inhibited, upon detection of the transient state of the differential.
To detect the transient state of the differential with high accuracy, it is required to not only detect the currently established operating state of the differential, but also obtain the subsequent operating state of the differential (currently desired operating state of the differential which will be established).